The transition from micro-scale to nano-scale devices, or MEMS to NEMS technology is a subject of intense scientific and engineering interest [1,2]. Most structural components in NEMS are synthesized from single crystal or polycrystalline silicon, or silicon carbide, silicon nitride, or silicon oxide thin films, with silicon being by far the most widely applied [7]. Silicon is easy to etch, elastically stiff and very hard. However, it has low fracture strength and poor wear characteristics, resulting in low reliability for moving components that contact other structures [8]. Silicon is also poorly suited to functional, electrically conductive applications.
MEMS/NEMS cantilever devices fabricated from thin film materials are the building block of many pieces of advanced equipment such as sensors and actuators. Low surface roughness and intrinsic stress, high hardness and modulus and good conductivity are some of the preferred features for high performance of these devices as the structural components of high-precision instruments.
Metallic films can provide better ductility and high electrical conductivity, but generally suffer from low strength and low modulus. Electroplated Ni films, while being commonly used for high aspect ratio micro-systems in MEMS, have not achieved widespread application for nano-scale structures. To date, the smallest feasible dimensions reported in a Ni-based single anchored cantilever were 10 μm long, 4 μm wide, and 210 nm thick [9]. Amorphous Au—Pd, which was successfully synthesized as a double-anchored cantilever with dimensions of 8 μm×100 nm×30 nm [10], is considered a much better candidate for NEMS applications.
The most widely used metallic thin film for functional applications is aluminum because of its compatibility with device processing. Unlike Si, Al is ductile and resists fracture. The critical problem with using Al for structural applications is that in pure form, or with minor alloying additions, the material is weak [11,12] and unable to withstand the stresses associated with fabrication of NEMS, such as device release steps. In addition, large surface roughness caused by extensive grain growth during deposition precludes its usage in most NEMS applications. A significant advantage of Al is that such Al-based materials could also carry significant electrical current. However, only with a markedly improved combination of strength and surface smoothness can Al-based materials become structural building blocks for NEMS devices.
One approach to obtain superior mechanical properties in Al films is to co-deposit Al with other alloy additions [1316] onto high temperature substrates to create either single phase supersaturated solid solutions with relatively coarse intermetallic precipitates (Al—Fe, Al—Ti or Al—Cr—Fe), or completely amorphous structures (Al—Fe).
There is a need in the art for metallic thin films which may be used in nano-scale devices, which are electrically conductive at room temperature and may otherwise mitigate the difficulties in the prior art.